Ousmane Sonko has slipped into a coma, his lawyers said on Tuesday.
Senegal's main opposition leader has been under arrest since July, and imprisoned in Dakar's hospital.
His health first started deteriorating when he started a hunger strike in August to protest against what he calls harassment by the state.
But Sonko, 49, resumed his hunger strike mid-October to support the opponents to President Macky Sall, and to call for freedom for political prisoners ahead of next year's elections.
His state of health is now "alarming" and "worsening" one of Sonko's lawyers, Ciré Cledor Ly, said.
He entered a deep coma on Monday.
His spokesperson, El Malick Ndiaye, wrote on social media that his supporters have asked the Senegalese authorities to end Sonko's detention, which he described as "unjust".
PASTEF, Sonko's party, which has officially been dissolved by the authorities, launched an "urgent appeal" to the president and to the religious authorities, "to not remain indifferent to this situation" and to use their influence to demand the immediate release of Sonko.
Sonko was jailed in July on charges including fomenting insurrection, criminal association in connection with a terrorist enterprise and undermining state security over incidents dating back to 2021.
This should render him eligible according to the authorities but a court in his stronghold of Ziguinchor ordered his reinstatement to electoral lists, a decision which paved the way for him to be a presidential candidate.
Sonko and his supporters continue to accuse President Sall of attempting to prevent him from running in the 2024 presidential elections.
Sall, who was elected in 2012 and will not run in the elections, denies the charges.
This weekend, presidential candidates in many parts of Senegal denounced a number of arrests of supporters as they collected political sponsorship, a crucial step in the run up to the February 2024 polls.